1. Field of the Invention
The present invention relates to a foot structure for a legged walking robot, and more particularly to a foot structure for a two-legged walking robot, which foot structure achieves a smooth and quick gait for walking of the robot, as with the gait of a human being.
2. Prior Art
There are known legged moving machines which are movable in a wide space range. One of the known legged moving machines is a two-legged walking machine or robot which can move freely in a small working environment, as disclosed in Japanese Laid-Open Patent Publications Nos. 62(1987)-97005 and 62(1987)-97006.
The conventional two-legged walking robots have rather sluggish gaits for walking, and are unable to move smoothly and quickly like human beings. Therefore, the known two-legged walking robots consume an undue amount of energy for walking and are subject to undesirable limitations on the range of jobs which are to be done by the robots.
The terrain under a two-legged walking robot may not necessarily be a flat horizontal terrain, but may be a rough terrain. It is desirable that the two-legged walking robot have foot soles well constructed to fit with the irregular surfaces of the terrain. If the foot soles were not shaped to fit with the rough terrain, then the foot soles would not be able to apply necessary drive forces to the terrain, resulting in a lack of sufficient reactive forces from the terrain. Thus, it would be impossible to control the posture and direction of the robot with accuracy, and the robot might stumble on the terrain.
When a foot of a two-legged walking robot lands on the terrain, it undergoes shocks or impacts due to reactive forces from the terrain. It is necessary to dampen such shocks or impacts in order to transmit required forces to the terrain for maintaining the desired robot posture or in order to vertically control the center of gravity of the robot in coordination with the periodic walking movements of the feet.
Some robot foot soles have curved front ends (corresponding to the tips of the toes) and curved rear ends (corresponding to the heels). The curved front and rear ends of the foot soles, however, make the robot unstable when the robot is held at rest and standing. When the foot soles land on or are lifted off the terrain, they are in linear contact with the terrain. If the terrain is not horizontal, then the foot soles contact the terrain unstably. Even if the terrain is horizontal, the foot soles contact with the terrain over a distance corresponding to the width of the foot soles. Consequently, when an undesirable moment is applied to turn the legs about the vertical axis, the foot soles are unable to produce a sufficient resistive force against the moment. As a result, the robot may turn from the forward direction into an unexpected direction.
The stability of a two-legged walking robot while it is being held at rest will also be of importance in the future when smaller and lighter actuators will be available. More specifically, while a two-legged walking robot is standing at rest, it is not desirable from the standpoint of saving energy to supply the actuators with an energy such as an electric current to keep the actuators locked in servo control at all times, however a discontinuation of the supply of the energy to the actuators impairs the static stability of the robot.
The stability of robots while in walking motion will be considered below. The stability of four-legged walking robots is much better than the stability of two-legged walking robots because the feet of the four-legged walking robots have a sufficient wide area for contact with the terrain and the ratio of the area of the feet to the height of the center of gravity is large. The two-legged walking robots are however poor in stability since the center of gravity is high, the area of the feet for contact with the terrain is small, and periods of time in which the robot is supported by only one leg while walking are inevitable. Sensors for achieving stable operation of the robot may include a visual sensor, a balance sensor (inclinometer), joint angle sensors, and sensors for detecting reactive forces applied from the terrain to the foot soles.
The sensors associated with the foot soles should preferably be capable of detecting the shape of an object which is stepped upon and the irregularity of the terrain, because the two-legged walking robot is required to move or walk in the same space range as human beings and there are many objects such as stairs, sills, electric wires in the floor, and surface irregularities in the space range.
The foot structure of a legged walking robot needs to fit well with small surface irregularities of the terrain on which the robot moves. The foot structure which fits well with small surface irregularities allows drive forces and reactive forces to be transmitted well between the feet and the terrain, so that the robot can be controlled in posture and direction. Sensors which are combined with the foot soles have to be covered for protection against damage. However, if the sensors are covered with a protective layer, then the sensitivity thereof is lowered, and the sensors do not operate reliably. If the sensitivity of the sensors is to be maintained at a desired level, then the protective layer over the sensors should be thinned down at the expense of better protection.